linux-kernel - Re: [PATCH 11/16] byteorder: provide a linux/byteorder.h with {be,le}_to_cpu() and cpu_to

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Date:	Wed, 28 May 2014 17:07:25 -0500
From:	Cody P Schafer <dev@...yps.com>
To:	Joe Perches <joe@...ches.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linux PPC <linuxppc-dev@...ts.ozlabs.org>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Subject: Re: [PATCH 11/16] byteorder: provide a linux/byteorder.h with
 {be,le}_to_cpu() and cpu_to_{be,le}() macros

On Tue, May 27, 2014 at 7:44 PM, Joe Perches <joe@...ches.com> wrote:
> On Tue, 2014-05-27 at 17:22 -0700, Cody P Schafer wrote:
>> Rather manually specifying the size of the integer to be converted, key
>> off of the type size. Reduces duplicate size info and the occurance of
>> certain types of bugs (using the wrong sized conversion).
> []
>> diff --git a/include/linux/byteorder.h b/include/linux/byteorder.h
> []
>> @@ -0,0 +1,34 @@
>> +#ifndef LINUX_BYTEORDER_H_
>> +#define LINUX_BYTEORDER_H_
>> +
>> +#include <asm/byteorder.h>
>> +
>> +#define be_to_cpu(v) \
>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint8_t) , v, \
>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint16_t), be16_to_cpu(v), \
>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint32_t), be32_to_cpu(v), \
>> +     __builtin_choose_expr(sizeof(v) == sizeof(uint64_t), be64_to_cpu(v), \
>> +             (void)0))))
>
> probably better to use BUILD_BUG instead of these 0 returns
>

They aren't 0 returns.

$ echo "int main(void) { int x = (void)0; return x; }" | gcc -x c -
<stdin>: In function ‘main’:
<stdin>:1:26: error: void value not ignored as it ought to be
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